Spray a 9-inch-by-13-inch baking dish with cooking spray.
Melt the butter in a large saucepan over medium heat until golden and fragrant, stirring frequently. Add the Peeps and salt, stirring until the Peeps are completely melted. Remove from heat.
Add the cereal and stir with a rubber spatula until combined. Press the mixture into the prepared pan, using a piece of greased parchment paper to press into the corners. Let cool for about 30 minutes, then cut into 24 squares. Top with a drizzle of melted white chocolate, if desired.